Melanocortins are known to have a broad array of physiological actions (Nakanishi, et al. Nature (1979) 278:423-427). Aside from their well known effects on adrenal cortical functions and on melanocytes, melanocortins have been shown to affect behavior, learning, memory, control of the cardiovascular system, analgesia, thermoregulation, and the release of other neurohumoral agents including prolactin, luetinizing hormone, and biogenic amines (De Weid et al. Methods Achiev. Exp. Pathol. (1991) 15:167-199; De Weid et al. Physiol. Rev. (1982) 62:977-1059; Gruber, K. A. et al. Am. J. Physiol. (1989) 257:R681-R694; Murphy et al. Science (1980) 210:1247-1249; Murphy et al. Science (1983) 221:192-193; Ellerkmann, E. et al. Endocrinol. (1992) 130:133-138; Versteeg, D. H. G. et al. Life Sci. (1986) 835-840). Peripherally, melanocortins have been identified to have immunomodulatory and neurotrophic properties, and to be involved in events surrounding partition (Cannon, J. G. et al. J. Immunol. (1986) 137:2232-2236; Gispen, W. H. Trends Pharm. Sci. (1992) 11:221-222; Wilson, J. F. Clin. Endocrinol. (1982) 17:233-242; Clark, D. et al. Nature (1978) 273:163-164; Silman, R. E. et al. Nature (1976) 260:716-718). Furthermore, melanocortins are present in a myriad of normal human tissues including the brain, ovary, lung, thyroid, liver, colon, small intestine and pancreas (Tatro, J. B. et al. Endocrinol. (1987) 121:1900-1907; Mountjoy, K. G. et al. Science (1992) 257:1248-1251; Chhajlani, V. et al., FEBS Lett. (1992) 309:417-420; Gantz, L. et al., J. Biol. Chem. (1993) 268:8246-8250; Gantz, L. et al, J. Biol. Chem. (1993) 268:15174-15179).
Recent studies have described an unexpected diversity of subtypes of receptors for the melanocortin peptides and determined that they belong to the superfamily of seven transmembrane G-protein linked cell surface receptors (Mountjoy, K. G. et al., Science (1992), supra; Chhajlani, V. et al., FEBS Lett. (1992), supra). Five melanocortin receptor subtypes have been cloned. The melanocortin-1 (MC1) receptor is found in melanoma cells, where it has a role in mediating pigmentation. The melanocortin-2 receptor (MC2-R or ACTH receptor) is found in the adrenal glands where it mediates the effects of ACTH (adrenocorticotrophic hormone). The melanocortin-3 receptor (MC3-R) is primarily found in the central nervous system (CNS) (Gantz, L. et al., J. Biol. Chem. (1993) 268:8246-8250), but its physiological function is still unknown. The melanocortin-4 receptor (MC4-R) has been found in the brain, where it is widely distributed in several areas, including the cortex, thalamus, hypothalamus, brain stem, and spinal cord (Gantz, L. et al. J. Biol. Chem. (1993) 268:15174-15179; Mountjoy, K. G. et al. Mol. Endocrinol. (1994) 8:1298-1308). MC4-R has recently been related to weight homeostasis. MC4-R “knock out” mice have been shown to develop obesity (Huszar et al. Cell (1997) 88:131-141). The feeding behavior leading to the obesity can be inhibited by injection of MSH peptides (Vergoni et al. Neuropeptides (1986) 7:153-158; Vergoni et al. Eur. J. Pharmacol. (1990) 179:347-355; Fan et al. Nature (1997) 385:165-168). The melanocortin-5 receptor (MC5-R) has a wide peripheral distribution and is believed to participate in the regulation of the exocrine gland function (Chen et al. Cell (1997) 91:789-798).